1. Field of the Invention
This invention relates to a plane light source unit, and more particularly to a plane light source unit which is applied advantageously as a backlighting means for a liquid crystal display device or the like.
2. Description of the Prior Art
Conventionally, a backlighting means for a liquid crystal display device or the like generally has such a construction that a lamp is located at a focus of a reflector having a parabolic section while a milk-white diffusing plate is located above the lamp. Various inventions have been made to optimize the configuration of a reflector, to adjust the diffusion coefficient of a diffusing plate, and so on.
Special constructions have also been provided which employ such a combination of a linear lamp and a light guide, one side end of which is adjacent to the lamp, and the configuration of the light guide is simulated by approximation to a point source and worked into a curved surface so as to emit the light in a particular direction, or the thickness of the light guide is varied along the direction of light, or else a lenticular is used wherein the prism angle is varied in accordance with the distance from the lamp, or otherwise such configurations as described above are combined suitably. With approximation to a point source, a light path can be simulated in almost every case, and it is possible to change the configuration of a light guide in accordance with the distance in the light direction depending upon such simulation. Such proposals have been made in a large number of patents and utility model registrations. Such plane light sources are disclosed, for example, in U.S. Pat. Nos. 4,126,383, 4,043,636, 4,059,916, 4,373,282, 4,285,889, 4,252,416, 3,546,438, 4,642,736 and 4,648,690 and Japanese Patent Laid-Open No. 62-278505.
While almost all of plane light sources are designed to emit light as uniformly as possible in all directions, it may be sometimes desired to concentrate emitted light in a particular direction depending upon application of a plane light source.
For example, in the case of application to a liquid crystal color TV set for personal use having a comparatively small viewing angle, it is required to emit the light only in a particular direction and make the amount of emitted light as uniform as possible over an entire emitting surface. FIG. 1 shows general construction of a liquid crystal color TV set. Referring to FIG. 1, reference numeral 1 denotes a liquid crystal display, 2 a body of the liquid crystal color TV set, 3 a normal line to the liquid crystal display 1, and 4 an eye of an observer. With the arrangement of the type mentioned, the liquid crystal display 1 is tilted upwardly at an angle of about 45 degrees with respect to the body 2 and is normally observed by an observer in a direction at an angle of about 15 degrees with respect to the normal line 3. Accordingly, a backlighting means which presents a higher luminance within a particular angular range indicated by X than at any other angular position would be advantageous in that the amount of emitted light can be concentrated to the particular angular range. In particular, the luminance of such a plane light source will exhibit a maximum value in a desired direction, which may be several times greater than a luminance value of an alternative plane light source of the type which emits light uniformly in all directions. Accordingly, if the backlighting means is used in a display device which has a narrow viewing angle in a particular direction, the display device can present a high luminance with a small power consumption.
However, a point source is not used as the light source for a backlighting means of the liquid crystal color TV set shown in FIG. 1 or the like except very rare special cases where the display has a small area. A light source actually used is generally such a volume light source as a fluorescent lamp which cannot be regarded as a point source and is very low in coincidence of approximation to a point source. Accordingly, while such plane light sources as have been proposed in the prior art have accurate and complicated configurations and require a high production cost, it is difficult for them to achieve such desired characteristics as described above.
Besides, light emitted by such a volume light source as a fluorescent lamp is diffused light and non-directive. In a strict sense, it is very difficult to assure a desired directivity using a diffused light emitting source.
Further, in order to obtain a plane light source of small size, it is preferable to make its greatest thickness substantially equal to the diameter of a light source lamp. However, a plane light source device of the type wherein a reflector having a parabolic section is disposed below a lamp as described hereinabove has a thickness substantially equal to twice to four times the diameter of the lamp. Accordingly, the light source device just mentioned cannot meet the requirement of small size.